Assessing the mobility of minerals in magma chambers
Abstract
The evolution of magma chambers at all scales is driven by thermal evolution as well as internal fluid dynamic and crystal processes. Crystal mobilization is also observed in porphyritic lava flows and crystal cargos found in some magmatic intrusions. In this study we observe the various forces that may be acting on minerals that are growing within magma chambers and assess whether they may cause mobilization. Particle mobility depends upon the direction and magnitude of the forces that are acting on a crystal, as well as the geometry and structure of the bed. Particles at the fluid-pile interface can be mobilized if they are not geometrically trapped, while the majority of the particles which compose the pile are trapped by a column of crystals above them and can only be mobilized once the crystals overhead are mobilized. Mobilization of crystals occurs when the vertical uplift forces overcome the resistive forces. These uplift forces may come from buoyant fluid flow or from volatiles. Resistive forces come from cohesion of crystals within the pile and growth of grains. Experiments have been performed driving convection thermally within a tank filled with fluid and a particle pile composed of two types of inert, spherical particles of the same size, show the different styles of mobilization when particles are exposed to forces by convecting fluid. The two particles types have different density (one closely matched to the liquid and one of increased density) and are mixed with different density profiles. The different piles show different mobilization styles depending on the mixing regime, mixing ratios (ratio of light to heavy particles), the density contrast between fluid and particle and the temperature difference. The mobilization regimes observed within our experiments are rolling, puffs and fountains, with the degree of mobilization increasing as more low density particles are brought closer to the interface. When light particles are overlain by heavy particles no mobilization is observed until the thickness of the high-density layer is sufficiently thin. When the dense crust is less than this critical thickness, the light particles can breakthrough and erupt over the dense layer. We can therefore use our knowledge of the forces acting on the particles grains and the structure of the pile to reinterpret magmatic cumulates.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.V33B3119G
- Keywords:
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- 1020 Composition of the continental crust;
- GEOCHEMISTRYDE: 1021 Composition of the oceanic crust;
- GEOCHEMISTRYDE: 1031 Subduction zone processes;
- GEOCHEMISTRYDE: 8439 Physics and chemistry of magma bodies;
- VOLCANOLOGY